1. Field of the Invention
The present invention relates to a wire dot printer head and a wire dot printer using this wire dot printer head, and more particularly to a wire dot printer head having a structure in which an armature is urged by a coil spring in a direction away from a core and a wire dot printer using this wire dot printer head.
2. Discussion of the Background
There has been known a wire dot printer head wherein an armature with a printing wire coupled thereto is pivoted between a printing position and a stand-by position, and when the armature is pivoted to the printing position, a tip of the wire is brought into collision with a printing medium to effect printing. In a certain wire dot printer head of this type, there has been proposed a device wherein a magnetic flux is produced by a coil around the armature, that is to be pivoted, for forming a magnetic circuit that causes the armature to be attracted from a stand-by position to a printing position to effect printing.
In the wire dot printer head of this type, the armature is pivotably supported in the direction away from the core, around which a coil is wound, with the pivot shaft as a center. The armature is urged by a coil spring, serving as an urging member, toward the direction away from the core. The armature described above has an arm supporting the printing wire. The coil spring comes in contact with this arm, wherein the armature is urged in the direction away from the core (see JPU Hei-5(1992)-2639). In the technique disclosed in this publication, a plastic member is provided at the end section of the coil spring for preventing an abrasion at the contact section of the coil spring and the arm, wherein the coil spring is configured to come in contact with the arm via the plastic member.
The armature is required to be violently pivoted as many as 2500 times per second between the printing position and the stand-by position with a recent increased printing speed. Therefore, in case where a winding end 100a of a coil spring 100 comes in contact with the side face of an arm 101 as shown in FIG. 6, the winding end 100a protrudes toward the arm 101 from the contact position 102 where the coil spring 100 comes in contact with the arm 101, thereby gradually scraping the side face of the arm 101. Finally, the arm 101 is broken from the contact position. Further, a winding end 100b of the coil spring 100 comes in contact with a coil supporting member 103 that supports the coil spring 100, whereby it gradually scrapes the coil supporting member 103 during the printing. This causes an unstable urging operation of the coil spring 100, resulting in that high-speed printing is impossible.
On the other hand, even in case where a plastic member is provided at both ends of the coil spring as in the technique disclosed in JPU Hei-5(1992)-2639, the winding end of the coil spring scrapes the plastic member during the printing. Therefore, the plastic member is broken from its contact section. Consequently, the winding end of the coil spring comes in contact with the side face of the arm, gradually scraping the side face of the arm. Further, the broken plastic member hinders the urging operation of the coil spring. Moreover, the urging operation of the coil spring is not stabilized since it is hindered by the weight of the plastic member, resulting in that high-speed printing is impossible.